Recent advances in the medical arts have enabled enzyme replacement therapies (ERTs) for a number of metabolic diseases. Those diseases include Gaucher, Krabbe, Fabry and Pompe diseases, as well as various mucopolysaccharidoses (MPS) and hypophosphatasia (HPP).
HPP is an inherited metabolic disorder that features rickets or osteomalacia caused by deficiency of tissue-nonspecific alkaline phosphatase (TNSALP; EC 3.1.3.1). TNSALP is an ubiquitous, cytosol-insoluble plasma membrane-bound enzyme. The human enzyme contains 524 amino acid residues, and the sequence is available on the UniProtKB/Swiss-Prot data base under the designation “PPBT-Human P05186”.
Hypophosphatasia is an inherited metabolic disorder of defective bone mineralization caused by deficiency of the TNSALP. Clinical severity is remarkably variable, ranging from death in utero to merely premature loss of dentition in adult life. Despite the presence of TNSALP in bone, kidney, liver, and adrenal tissue in healthy individuals, clinical manifestations in patients with hypophosphatasia are limited to defective skeletal mineralization that manifests as rickets in infants and children and osteomalacia in adults.
In the most pernicious form of hypophosphatasia, the perinatal lethal variant, profound skeletal hypomineralization results in caput membranaceum with shortened and deformed limbs noted. Some affected neonates survive for several days or weeks. The neonates often succumb to respiratory failure brought on by pulmonary hypoplasia and structural failure of the weakened skeleton from demineralization.
Prenatal therapy for inborn errors of metabolism remains an unmet challenge. Most cases of perinatal HPP with severe skeletal hypomineralization result in stillbirth or postnatal lethality. Over 50% of the patients with an infantile form of the condition die within the next few months. Advanced ultrasonography made in utero diagnosis of severe skeletal dysplasia including HPP feasible.
Acidic amino acid (AAA) oligopeptides bind specifically to bone matrix, hydroxyapatite calcium site, and tagging a therapeutic agent, namely TNSALP, with AAA markedly enhances delivery of the agent to bone [Nishioka et al., (2006) Mol Genet Metab, 88 (3):244-255]. ERT has been successively shown by a deca-Asp (D10)-tagged TNSALP (sALP-FcD10) in a HPP murine model (similar to the infantile form in human patients) to lead to marked clinical and pathological improvement [Milian et al., (2008) J Bone Miner Res, 23 (6):777-787]. The sALP-FcD10 enzyme is now used in Phase I and II clinical trials for HPP patients, resulting in substantial reversal of bone hypomineralization [Whyte et al., Hypophosphatasia: Treatment of Life-Threatening Disease Using Bone-Targeted Human Recombinant Tissue Non-Specific Alkaline Phosphatase (2009) ACR/ARHP Scientific Meeting, found at acr.confex.com/acr/2009/webprogram/Paper12463.html. This sALP-FcD10 enzyme also includes the Fc region of human IgG at the C-terminus of the enzyme for purification purposes.
Recently, Grubb et al. showed that Fc-conjugated β-glucuronidase, infused into pregnant mice, was transported across the placenta, indicating a potential application for prenatal therapy [Grubb et al., (2008) Proc Natl Acad Sci U.S.A., 105(24):8375-8380]. β-glucuronidase (GUS) is a cytosol- and water-soluble, sialic acid-containing glycoprotein enzyme that catalyzes breakdown of complex carbohydrates. Maternal IgG is transported transplacentally by the neonatal Fc receptor, which recognizes the Fc domain of IgG and mediates transcytosis from maternal to fetal circulation.